This invention relates to a driving wheel slip control system for vehicles, and more particularly to a driving wheel slip control system comprising a plurality of electronic control units and having a failsafe function for coping with an abnormality thereof.
Conventionally, there have been proposed control systems, such as a driving wheel slip control system, which are each composed of a plurality of electronic control units (hereinafter referred to as "ECU's") connected with each other by signal lines. For example, the present assignee has already proposed by Japanese Provisional Patent Publication (Kokai) No. 2-157439, a driving wheel slip control system which has an ECU (hereinafter referred to as "TSC-ECU") for detecting a slip state (degree of a slip) of driving wheels of a vehicle, and an ECU (hereinafter referred to as "ENG-ECU") for controlling output of a prime mover for driving the driving wheels based on a slip level signal which is supplied from the TCS-ECU and indicates the slip state of driving wheels in terms of an analog value, i.e. a continuously variable slip level.
A method (hereinafter referred to as "the first method") is widely known, which determines that a signal line of the above described driving wheel slip control system for transferring the slip level signal or a part or parts related thereto is abnormal when the slip level represented by the slip level signal is not within a predetermined range defined by upper and lower limit values.
Further, another method (hereinafter referred to as "the second method") is also known, which employs, in addition to the slip level signal, a slip status signal which is a binary signal representing whether or not the driving wheels are in a predetermined slip state (e.g. a slip state in which the slip level represented by the slip level signal is higher than a predetermined value), and which is also transferred from the TCS-ECU to the ENG-ECU. According to the second method, when a state in which the slip level signal and the slip status signal are contradictory to each other is detected, it is determined that one or both of signal lines for transmitting these signals is/are abnormal. More specifically, if the slip status signal indicates that the driving wheels are not in the predetermined slip state, whereas the slip level signal indicates that the driving wheels are in the predetermined slip state, or vice versa, it is determined that abnormality exists.
According to the first method, so far as the slip level represented by the slip level signal is not held outside the range defined by the upper and lower limit values, the signal line for transmitting the slip level signal is not determined to be abnormal. Therefore, the first and second methods can be combined, whereby it is possible to determine that the signal line for the slip status signal is abnormal, if the slip level signal and the slip status signal are contradictory to each other and at the same time the slip level indicated by the slip level signal is not held at a high level higher than the upper limit value nor at a low level lower than the lower limit value.
However, if some kind of oscillating signal, for example, accidentally intrudes into the signal line for the slip level signal, when the driving wheels are actually not in the aforementioned predetermined slip state and the slip status signal correctly represents the actual slip state of the driving wheels, there can be the case where the slip level indicated by the slip level signal assumes an intermediate value between the upper and lower limit values, to falsely represent the predetermined slip state of the driving wheels (hereinafter referred to as "the intermediate value hold abnormality"). In such a case, according to the above combination of the first and second methods, it is determined that the signal line for the slip status signal is abnormal since the slip level indicated by the slip level signal is not held at the high level nor at the low level. Thus, it is impossible to detect the intermediate value hold abnormality resulting from the aforementioned abnormality of the signal line for transmitting the slip level signal.
Further, if driving wheel slip control is stopped immediately when abnormality of the slip level signal or the slip status signal is detected by either or combination of the first and second methods, there can occur a sudden rise in the torque of driving wheels, to thereby degrade the driveability of the vehicle.
On the contrary, if driving wheel slip control is continued even though an abnormality has been detected, there is an undersired possibility of the control being carried out based on information (i.e. the slip level signal or the slip status signal) which does not correctly represent the actual slip state of driving wheels.
Further, in addition to cases where an abnormality is detected as described above, if the engine coolant temperature, for example, is very high such that the engine can overheat if the amount of fuel supplied to the engine is decreased, driving wheel slip control by decreasing the amount of fuel should be inhibited. If such a hot state occurs while driving wheel slip control is being carried out, and driving wheel slip control is immediately stopped, the same incovenience as described above (i.e. degradation of the driveability of the vehicle) results.
It is also generally known that in the above-mentioned control system having two ECU's, one of the two ECU's checks abnormality of the other and the one sends the result of checking to the other. In the aforementioned driving wheel slip control system proposed by the present assignee as well, the ENG-ECU checks abnormality of the TCS-ECU (or the signal line connecting the ECU's with each other) based on the slip level signal, and sends the result of checking to the TCS-ECU.
However, depending on operating conditions of the prime mover, there are cases where driving wheel slip control should preferably be inhibited (e.g. when the warming-up of the engine is not completed, or when the engine coolant temperature is extremely high). According to the above-mentioned manner of communication between the ECU's, even in such cases, no information is sent from the ENG-ECU to the TCS-ECU so long as no abnormality of the TCS-ECU is detected by the ENG-ECU. Therefore, even in cases where driving wheel slip control should not be carried out, which makes calculations by the TCS-ECU unnecessary, the TCS-ECU carries out calculations nevertheless, which leaves room for improvement of the driving wheel slip control system.
Further, in the aforesaid proposed system, the TCS-ECU is adapted to warn the driver of a detected abnormality of the driving wheel slip control system, e.g. by lighting a lamp. In such cases, if no abnormality of the TCS-ECU etc. is detected and at the same time driving wheel slip control should not be carried out, it is impossible to inform the driver of the state of the system in which driving wheel slip control cannot be carried out.